Currently available vertically capable aircraft (VTOLs) are generally denied permission for routine powered terminal operations (e.g., take-off, low altitude climb, landing, etc.) in populated, built-up areas for one or more of four reasons: safety, noise, exhaust emissions, or outwash velocity. Further, current rotary-wing VTOLs, except for very advanced tilt rotor aircraft, cannot compete with similar payload-class, fixed-wing, propeller-driven aircraft in speed and range when unrestricted expansive take-off and landing facilities and climb corridors are conveniently available at both ends of a mission. So the simultaneous attainment of radically improved terminal safety, tolerable noise and fumes, modest outwash velocity and competitive fixed-wing speeds, efficiencies, and ranges would enable rotary-wing aircraft to dominate the current light aircraft market, subject to price differentials, and open up the vast denied market for terminal operations in built-up areas. Two other factors, though not essential to correct the above rotary-wing shortfalls, add to the market expansion potential for the subject electrically-powered rotor craft: (1) independence from logistically burdensome fuels (e.g., JP, H2, etc.) at light-duty bases, particularly in built-up areas, and (2) fully autonomous flight control/management to relieve the stiff requirement for specialized pilot proficiency, thus eliminating another disincentive for vertical aircraft ownership/operation.
Although numerous low-performance electric fixed-wing aircraft have been built, the only widely publicized claims to an electric tilt rotor aircraft are made by FALX AIR™ Hybrid Tilt Rotor. To the degree that popular descriptions are accurate: (1) the configuration is a low aspect ratio tilt-wing, not a tilt-rotor; (2) the batteries in the FALX AIR are supplemental to the internal combustion engine to assist Hover-Out-of-Ground-Effect (HOGE) and climb and do not provide separate full HOGE power; hence, the FALX AIR lacks fully redundant power in the dead man zone for silent, safe takeoff and landing in built-up areas; (3) the dual electric motors/nacelle are insufficient at this moderately high disk loading to supply HOGE with one-propulsion-motor-inoperative (OPMI), thus severely compromising safety in built-up areas; and (4) the FALX AIR makes no pretense of basing-independence allowing all-electric operation for basing in the absence of conventional logistic fuels.
Similarly, the Aurora Flight Science's™ Excalibur concept VTOL electric hybrid is not a tilt-rotor configuration, but rather a direct thrust turbofan, 70% of vertical lift, with supplemental electric ducted fan lift during HOGE.
Four recent advances in disparate technologies can synergize to enable efficient electric tilt-rotor VTOL aircraft. Tilt-rotor aerodynamic, structural, and propulsive efficiencies have improved. Extremely flight-efficient tilt-rotor aircraft, far beyond the V-22's anemic lift-to-drag ratio, low propulsion efficiency, and high structural weight fraction result in more than 2× the V-22's specific payload×range. Electric motor power densities have increased. High-performance, light-weight electric motors and generators can have more than three times the power-density of motors being introduced in electrically propelled automobiles. Battery energy densities have also increased and can provide energy densities of 100, 200, 300, or even up to 400 W-hrs/kg. Furthermore, autonomous flight control and management systems have dramatically improved. For example, autonomous flight control and route/ATC management with pilot override, which allow for totally autonomous flight from takeoff to landing have been demonstrated in the A-160 Hummingbird.
All of the above individual subsystem elements for a new electrically-powered tilt-rotor VTOL (E-VTOL) have already been separately demonstrated: (1) Hardware has been demonstrated with prototypes of very high performance electric motors/generators, small/light/low-sfc turbines, moderately high performance lithium batteries, variable speed rigid rotors, light weight all-carbon structures, and autonomous flight/management of rotary wing VTOLs. (2) Extensive vetting by independent parties of related aerodynamically efficient tilt-rotor airframe designs (though not with electric propulsion architectures) has testified as to the practicality of the assumed aerodynamics and weights. (3) Finally, the very high-performance lithium batteries necessary for the purebred battery electric architectural variant are at the bench chemistry stage within the National labs and less visibly with private firms, thus developable with expected vigor.
What has yet to be appreciated is that the above advances can now be combined to realize many new capabilities that address issues with the known art. The contemplated E-VTOL aircraft have tolerable noise, zero emissions, or acceptable outwash velocity necessary for powered terminal operations in populated, built-up geography. An E-VTOL aircraft has vertical flight safety improvements to bring rotary-wing aircraft into parity with fixed-wing competitors (e.g., factor of 10 reductions in accidents per flight-hour) and makes vertical flight politically compatible with terminal operations in built-up areas, such as elimination of the “dead man's zone”. Electrically-powered, vertically-capable aircraft can have market-competitive speed and range relative to current personal, executive, light cargo, public safety, and military fixed-wing, propeller-driven aircraft below 20,000 lb gross weight. Such aircraft also have the benefit of basing-independence from conventional on-site liquid fossil fuel support for short range operations where only electrical power would likely be required for recharging batteries. The aircraft also have naturally low infra-red and acoustic signature in terminal operations where combat threats are most prevalent. Contemplated designs also eliminate a requirement for a two-speed gearbox or mechanical cross shafting that would ordinarily be necessary for optimized vertical lift, horizontal cruise rotor RPM, and safe vertical terminal operations when separate rotor nacelles are driven by conventional turbine engine mechanical drive trains. Designs can also include non-tilting sustainer engines in the electric hybrid which avoid lubrication problems and engine design specialization in typical “engine-in-nacelle” tilt-rotor aircraft. Additionally electric hybrid VTOL (E-VTOL) have a wide flexibility in choice of sustainer energy source types or sizes within the same airframe to suit the desired cruise speed and altitude with no change in rotor electric drive motors which are sized for vertical flight and hence over-powered for all but highest speed cruise.
The above advanced capabilities can be achieved using multiple electric motors to drive each rotor through one or more fixed reduction gearboxes and a choice of at least three power supply architectures, all of which enable full redundancy in both rotor drive motors and electric power supply for safe, hover-out-of-ground-effect (HOGE) in built-up areas. All three are purely electric during quiet, emission-free operations in built up areas. A heavy hybrid can be entirely electric, hence basing-independent, for short range operations (e.g., less than 50 nautical miles). A purebred battery architecture can be innately all-electric for full flight range (e.g., greater than 200 nm). A light hybrid offers full range (e.g., on the order of 1000 nm) flight, but can require traditional logistic fuel availability under normal basing conditions even though it retains quiet, safe, all-electric terminal operations capability. All designs benefit from fully autonomous flight control with pilot override to reduce or eliminate pilot skill requirements and further improve safety of this inherently complex vertical lift aircraft.
Therefore, there remains a considerable need for methods, systems, and configurations for providing VTOL tilt-rotor aircraft.